This invention relates to an automatic transmission of an automobile, particularly to a controlling hydraulic circuit preferably applicable to a control apparatus of a multistage automatic transmission comprising a main shifting unit and a sub-shifting unit, and more particularly to a control mechanism of a hydraulic servo for a friction engaging element.
Generally, an automatic transmission includes a torque converter and a planetary gear shift mechanism. The gear shift mechanism comprises an overdrive (O/D) planetary gear unit or underdrive (U/D) planetary unit, a front planetary gear unit and a rear planetary gear unit. The gear shift mechanism offers a four forward stages and one gear stage due to engagement or retainment of predetermined elements by clutches or brakes (friction engaging elements) controlled by a manual valve, two solenoid valves and three shift valves.
Heretofore, the aforementioned friction engaging element has been controlled by a hydraulic servo in which oil pressure is fed or drained according to a change-over operation of the manual valve and shift valve. Generally, the hydraulic servo is provided with an accumulator disposed in a parallel relation thereto. The engaging characteristics of the respective friction engaging elements are set according to a servo pressure of the hydraulic servo based on the setting hydraulic characteristics of the accumulator.
By the way, a piston of the hydraulic servo is required to move over a predetermined stroke until friction plates of the friction engaging elements contact with each other and a pressure, enough to bear a predetermined torque is realized. However, the stroke of the piston often causes a delay in the engagement of the friction engaging elements. In addition, a shifting shock arises due to gap of shifting time caused by the delay of engagement.
As a countermeasure to the shifting shock due to the delay of engagement, modified orifices, accumulator springs and accumulator pistons have been used. However, the modification of the orifices and accumulators often result in a sacrifice of the proper hydraulic characteristics.
On the other hand, as disclosed in a Japanese Laid-open patent application No. 57-37140, there has been proposed by the present applicant an automatic shift control apparatus for obtaining six forward stages of shifting, wherein three solenoid valves and three shifting valves are provided, an overdrive planetary gear unit functioning as a sub-shifting unit is used in combination with a main shifting unit comprising front and a rear planetary gear units.
By the way, the multistage automatic transmission comprising the sub-shifting unit and main shifting unit obtains six stages of shifting through a combination of two stages of the overdrive and direct connection of the sub-shifting unit and first speed, second speed and third speed of the main shifting unit. At this time, a situation occurs in which one of the shifting units is down-shifted while the other is up-shifted. For example, when kicked down, the sub-shifting unit is up shifted from the directly connecting state to the overdrive and a simultaneously the main shifting unit is down shifted from second speed to first speed, thus shifting from third speed to second speed in total. In the foregoing case, according to the conventional control apparatus, since no countermeasure is taken to complete both the shifting operations at the same time, there is the possibility that one of the shifting operations is effected first and then the other shifting operation is effected. For example, when the up-shifting operation is effected first, the transmission is first increased (to fourth speed) in speed extensively, and thereafter is decreased (to second speed) in speed extensively. On the other hand, when the down-shifting operation is effected first, the transmission is first decreased (to first speed) in speed extensively and thereafter increased in speed. Due to the foregoing, there occurs a kick-down which diminish the acceleration efficiency. In addition, there occurs a large shock which again poorly effects the shift feeling.